1. Field of the Invention
This invention relates to cardiac pacing systems and, more particularly, to dual chamber pacing systems designed to optimize the sensing of tachycardia atrial senses while rejecting far field R wave senses.
2. Description of the Prior Art
The advantages of dual chamber pacing are now well recognized in the art. Since the dual sense-dual pace DDD pacemaker became commercially available, it has gained favor for the reason that it compensates for many of the disadvantages of other pacemaker modes. The classic DDD pacemaker is described in U.S. Pat. Nos. 4,920,965, 4,539,991 and 4,554,921, incorporated herein by reference. In such a DDD pacemaker, or a VDD pacemaker, atrial events are sensed, allowing natural or physiological P-waves to synchronously control the ventricular pacing rate. An area of dual chamber pacemakers that has received a great deal of attention in the pacemaker industry is that of dealing with high atrial rates. If high atrial rates can be recognized, the pacemaker can optimize synchronous tracking but also be able to recognize atrial tachycardia and take appropriate responses when such tachycardia occurs. It is known, of course, to provide a Wenckebach-type response which provides a degree of synchronization for high rates and maintains the ventricular stimulus rate within an upper rate limit.
Another means of responding to high rate atrial signals, whether or not premature atrial contractions, is to extend the post-ventricular atrial refractory period (PVARP). Likewise, the atrial refractory time can be extended past the time of anticipated retrograde P-waves to avoid sensing of such retrograde P-waves, so as to avoid pacemaker-mediated tachycardia. It is also known to vary PVARP as a function of sensor data. See also Funke, U.S. Pat. No. 4,920,965, where the atrial pacing timeout and effective PVARP are varied as a function of certain sensed signals. However, the extension of PVARP or the atrial refractory period lowers the upper rate limit for sensing atrial signals, and thus reduces the capability of the pacemaker to provide synchronous operation in response to high rate atrial heartbeats which may yet be physiological.
Another approach is to reduce to PVARP as much as possible, in order to enhance detection of high rate atrial signals. However, this makes a VDD or DDD pacemaker more vulnerable to far field R wave senses (FFRS). An FFRS following a ventricular pace (VP) is known to occur within an interval of up to about 180 ms from the VP, while an FFRS following a ventricular sense (VS) occurs within a shorter interval (e.g. 0-100 ms). An extension of PVARP to &gt;100 or 180 ms, in order to blank or reject a potential FFRS, is contrary to the objective of minimizing PVARP as much as possible in order to be able to sense, or "see" as many high rate atrial signals as possible.
U.S. Pat. No. 5,247,929, assigned to the same assignee as this invention and incorporated by reference, discloses a dual chamber pacemaker which includes a VA conduction test for concurrently testing for the conditions of retrograde conduction (RC) and FFRS. In this patent, the pacemaker changes the AV or VV interval to a predetermined interval over a number of pacemaker cycles, and determines whether the VA interval remains substantially unchanged, i.e., whether the VA intervals are substantially equal. When VA interval equality is determined, the pacemaker checks the measure of VA interval with stored RC and FFRS criteria to determine whether either can be verified. However, for FFRS detection, this procedure demands an extended period of atrial sensing following the ventricular pacing. Such condition is generally met in the case of retrograde conduction, but often is not found in the case of far field sensing. Accordingly, what is needed is a more effective technique which captures (identifies) and suppresses incidentally sensed far field signals, while blanking of real atrial signals (P-waves) is avoided as much as possible.